1. Field of the Invention
The present invention relates in general to electrochromic devices, and more particularly, to normally operating, color-stabilized electrochromic devices having an electrochromic medium comprising one or more redox buffers, which serve to substantially preclude the formation of undesirable residual color within the electrochromic medium while in its high transmission state.
2. Background Art
Electrochromic devices have been known in the art for several years. While the utilization of electrochromic devices, such as electrochromic mirrors, has become increasingly popular among, for example, the automotive industry, the development of undesirable residual color within the electrochromic medium of such electrochromic devices remains problematic.
Indeed, when a sufficient electrical potential difference is applied across the electrodes of a conventional electrochromic device, the electrochromic medium becomes intentionally colored (i.e. a low transmission state) inasmuch as one or more of the anodic and the cathodic materials are oxidized and reduced, respectively. Specifically, the anodic materials are oxidized by donating one or more electron(s) to the anode and the cathodic materials are reduced by accepting one or more electron(s) from the cathode.
For most commercially available electrochromic devices, when the electrical potential difference is removed or substantially diminished, the anodic and/or cathodic materials return to their zero-potential or unactivated state, and in turn, return the electrochromic medium to a predetermined state, which is conventionally colorless, nearly colorless, or intentionally tinted (i.e. a high transmission state). The application and removal of an electrical potential difference is conventionally known as a single cycle of the electrochromic device.
Scientists have observed that over a period of cycles and/or time, during normal operation of the electrochromic device, the electrochromic medium sometimes does not acceptably return to a predetermined state. In some instances, even in the absence of an electrical potential difference, a portion of the anodic and cathodic materials may be oxidized or reduced respectively, thereby forming residual color from the oxidized and/or reduced materials. The residual oxidized anodic materials or the residual reduced cathodic materials of the electrochromic medium can result in an undesired residual coloration of the electrochromic medium.
Factors that are believed to facilitate the formation of the undesired residual oxidized anodic and/or reduced cathodic materials include, among other things, thermal and/or photochemical decomposition of one or more of the medium materials, and/or the permeation of water and/or oxygen into the electrochromic medium.
It is therefore an object of the present invention to provide an electrochromic medium with one or more redox buffers which remedy the aforementioned detriments and/or complications associated with controllably maintaining a predetermined color of an electrochromic medium (i.e. colorless, nearly colorless, or intentionally tinted) while in a high transmission state relative to an electrochromic medium without the redox buffer.
The present invention is directed to the use of one or more redox buffers in an electrochromic medium. To act as a redox buffer, a material will exhibit an electrochemical reaction within a range bounded by the first oxidation potential of the principal anodic material, typically the anodic electrochromic material, and the first reduction potential of the principal cathodic material, typically the cathodic electrochromic material. The material may exhibit the electrochemical reaction initially, or alternatively may generate a species that exhibits the electrochemical reaction after a chemical reaction that follows oxidation or reduction. In this case, the redox potential corresponding to the oxidation or reduction process may lie outside of the above-identified range.
The present invention is also directed to an electrochromic device comprising: (a) at least one substantially transparent substrate having an electrically conductive material associated therewith; and (b) an electrochromic medium having a predetermined color while in a high transmission state, wherein the electrochromic medium comprises: (1) an anodic material and a cathodic material, wherein both of the anodic and cathodic materials are electroactive and at least one of the anodic and cathodic materials is electrochromic; (2) a redox buffer; and (3) means associated with the redox buffer for controllably maintaining the predetermined color of the electrochromic medium while in the high transmission state relative to an electrochromic medium without the redox buffer.